1. Field of the Invention
The present invention relates to two wire line voltage thermostats and, more particularly, to such thermostats which are minimally affected by self generated heat and which provide power to the temperature responsive circuitry both during "off" periods and "on" periods of the thermostats.
2. Description of the Prior Art
Most electrical thermostats currently in use are low voltage thermostats. Low voltage thermostats, however, are costly to install because the low voltage outputs which they ssupply are insufficient to operate heat generating appliances. Thus, a contactor/relay is also included in the thermostat system for responding to the low voltage signal from the thermostat to switch line voltage to the heat generating appliance whereby the low voltage thermostat must be installed in two parts. One is the thermostat which is typically mounted on a wall of a living space within a residence, and the other is the contactor/relay which is typically installed on the heat generating appliance such as a furnace located in a basement of the residence.
Line voltage thermostats, on the other hand, reduce the complexity of installation normally associated with low voltage thermostats because line voltage thermostats do not require the contactor/relay. That is, line voltage thermostats can switch line voltage directly and, accordingly, can directly control the heat generating appliance. When line voltage thermostats are satisfied, that is when an ambient temperature sensed by the line voltage thermostat is at a setpoint temperature, the line voltage thermostat is open, and power is not supplied to the heat generating appliance. When the ambient temperature falls below the setpoint temperature, however, the thermostat closes, i.e., the thermostat's load controlling contacts are shorted, to supply power to the heat generating appliance. If the line voltage thermostat is an electronic thermostat, the closing of the load controlling contacts will bypass the electronics unless the thermostat is otherwise arranged to provide power during "on" periods of such thermostats.
Such arrangements can include running extra wires for powering the thermostat when the load is energized; this approach, however, involves extra installation complexity. Another arrangement is the use of a sizable capacitor to store energy during "off" times of the thermostats to be used by the electronics of the thermostat during the "on" periods of the thermostat. This arrangement, however, increases the cost of the thermostat itself and can create problems when the "on" periods of the thermostat far exceed the "off" periods in terms of length of time. Line voltage thermostats have also historically generated heat which has impaired the operation of the thermostat. Low voltage thermostats are, of course, subject to the same power considerations when such thermostats contain electronics which must be powered during "on" periods. Low voltage thermostats, in addition to using either extra wires for supplying power during "on" periods of the thermostat or a storage capacitor for storing power during "off" periods of the thermostat to be used during "on" periods, have alternatively used various power stealing arrangements. A power stealing arrangement is one which steals, during "on" periods of the thermostat, an amount of power insufficient to impair the functioning of the heat generating appliance but sufficient to power the electronics of the thermostat during such "on" periods.